Henderson–Hasselbalch equation

In chemistry, the Henderson–Hasselbalch equation describes the derivation of pH as a measure of acidity (using pK_a, the negative log of the acid dissociation constant) in biological and chemical systems. The equation is also useful for estimating the pH of a buffer solution and finding the equilibrium pH in acid-base reactions (it is widely used to calculate the isoelectric point of proteins).

The equation is given by:

Here, [HA] is the molar concentration of the undissociated weak acid, [A⁻] is the molar concentration (molarity, M) of this acid's conjugate base and pK_a is −log₁₀K_a where K_a is the acid dissociation constant, that is:

In these equations, A⁻ denotes the ionic form of the relevant acid. Bracketed quantities such as [base] and [acid] denote the molar concentration of the quantity enclosed.

For the standard base equation:

A second form of the equation expressed in terms of ${\displaystyle K_{\mathrm {b} }}$ where ${\displaystyle K_{\mathrm {b} }}$ is the base dissociation constant:

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